div.cover img {
    max-height: 100%;
    max-width: 100%;
}

.navbar {
    background-image: url("img/bgdanghang3.png");
    background-repeat: no-repeat;
    background-size: contain;

    font-size: 100%;
    height: 100%;
    width: 100%;
    display: inline-block;
    padding: 1em;
}

.navbar a {
    color: white;
    padding: 0;
    margin-left: .5em;
    float: left;
}

div.columns {
    display: flex;
}

div.columns > * {
    background-image: url(img/bgc1.jpg);
    background-repeat: no-repeat;
    background-size: cover;
    margin: 2em;
}